The present invention relates to a process for brominating fluorine-containing halogenated hydrocarbons by reaction with bromine.
Contrary to unsubstituted hydrocarbons or chlorine-containing hydrocarbons, the bromination of which can be effected at temperatures less than 300.degree. C., fluorinated hydrocarbons require a considerably higher temperature for bromination thereof. Thus, bromination of chloro-difluoromethane into bromo-chloro-difluoromethane by means of bromine is effected at 560.degree. C. according to the process described in the German Auslegelschrift No. 1,168,404. The bromination of trifluoromethane into bromo-trifluoromethane according to the German Pat. No. 1,155,104 is effected at a temperature of between 650.degree. and 800.degree. C. The bromination of 1,1,1-trifluoro-2-chloroethane into 1,1,1-trifluoro-2-bromo-2-chloroethane according to the German Pat. No. 1,113,215 is effected at about 500.degree. C.
An important disadvantage of these thermal bromination processes is the limited choice of reactor materials due to the necessarily high reaction temperatures. Thus, high temperature steels are not suitable and expensive special alloys having a high content in nickel are likewise only useful to a certain degree because of the highly corrosive behavior of elementary bromine and the hydrogen bromide formed within the reaction mixture. Materials based on silicates are also useless because of the presence of hydrogen fluoride within the reaction mixture.
A further disadvantage of the thermal bromination resides in the fact that, at the necessary high reaction temperatures, numerous side reactions take place like, e.g., chlorine-bromine exchange, splitting-off of hydrogen fluoride and cracking of the organic compounds under formation of numerous high boiling and, as well, tar- and coke-like products. These may cause considerable disturbances within the production process and may considerably lower the yield in desired raw products, whereby the economical value of the process is greatly decreased.
U.S. Pat. No. 2,658,086 discloses a process for brominating trifluoromethane into bromo-trifluoromethane by means of bromine in the presence of chlorine, yet without irradiation wherein the molar ratio between chlorine and bromine is not higher than 2. Yet this bromination occurs only at a temperature of about 450.degree. C. and the yield is only 36.6 mole percent.
The bromination of trifluoromethane by means of bromine under irradiation with a mercury vapor lamp, but without addition of chlorine has been disclosed (see Corbett, Tarr and Whittle, Trans. Faraday Soc. 59 (1963), p. 1615). Yet, the described bromination can be observed only at temperatures of above 275.degree. C.